Defenitions of Kingdoms:
(A domain in which something is dominant) "the untroubled kingdom of reason"; "a land of make-believe"; "the rise of the realm of cotton in the south" http://wordnetweb.princeton.edu/perl/webwn?s=kingdom
A government or country headed by a king or queen; monarchy
http://www.yourdictionary.com/kingdom
The Kingdom is God's Reign. "The Kingdom of God" means primarily the rule of God, the divine kingly authority.
http://www.biblestudy101.org/definitions/kingdom.html